JP7493481B2 - Ultrasound Imaging Device - Google Patents

Description

The present invention relates to an ultrasound imaging device.

The speed of sound in a living body varies depending on tissues such as fat and muscle, and also varies from person to person. An ultrasound imaging device transmits ultrasound from an array of ultrasound elements toward a living body, receives echo signals generated within the living body, and performs receive beamforming processing in which the resulting received signals (channel RF signals) are delayed by a delay time according to the distance between the receiving focus and each ultrasound element, and then added. The delay time during receive beamforming is generally calculated using a preset average value of the speed of sound of ultrasound propagating within the living body. However, if the set sound speed differs from the actual sound speed, the coherence between the echo signals of each channel decreases, leading to poor image quality.

In the technology described in Non-Patent Document 1, the channel RF signal obtained from the imaging target is temporarily stored, the sound speed used for receive beamforming is changed to multiple types, and the delay time for each sound speed is calculated. Using the calculated delay time, receive beamforming is performed to generate ultrasound images for each of the multiple sound speeds, and the focus degree of these images is evaluated. Based on the obtained focus degree, an optimal delay time is determined and an ultrasound image is generated. In this way, an ultrasound image can be captured at a sound speed suitable for the imaging target. The beamforming sound speed at which such an optimal image is obtained is called the average sound speed, etc., to distinguish it from the sound speed as the physical property value of tissue. If the physical property sound speed of the imaging target is uniform, the average sound speed and the physical property sound speed are the same, but if the imaging target has multiple physical property sound speeds, the average sound speed in the ultrasound propagation path is estimated as the optimal beamforming sound speed.

On the other hand, Non-Patent Document 2 discloses an adaptive beamforming technology that suppresses low-coherence artifacts and noise components by weighting the received signal according to its coherence.

David Napolitano, et.al,”Sound speed correction in ultrasound imaging, Ultrasonics”,Volume 44, p43-46(2006),ISSN 0041-624X S. M. Hverven, O. M. H. Rindal, A. Rodriguez-Molares and A. Austeng, "The influence of speckle statistics on contrast metrics in ultrasound imaging," 2017 IEEE International Ultrasonics Symposium (IUS), Washington, DC, USA, 2017, pp. 1-4

Since the imaging subject contains multiple tissues with different sound speeds arranged in a complex manner, if the degree of focus can be evaluated on a pixel-by-pixel basis, pixels with high coherence can be extracted, improving the robustness and accuracy of sound speed estimation and enabling high-quality imaging with reduced artifacts and noise.

However, the technology in Non-Patent Document 1 can generate ultrasound images for multiple types of sound speeds and evaluate the degree of focus for each image, but cannot evaluate the degree of focus on a pixel-by-pixel basis.

On the other hand, adaptive beamforming technology requires a large amount of calculations because it uses correlation calculations between the echo signals of each channel, and it is necessary to install dedicated adaptive beamforming circuits and software on the front end of the device.

The objective of the present invention is to calculate the coherence index of the received signal for each pixel with a small amount of calculation, and to obtain high-quality ultrasound images.

In order to achieve the above object, the ultrasound imaging device of the present invention has a memory for storing received signals, an ultrasound image generating unit, a sound speed variation image generating unit, and a coherence index calculation unit. The memory stores received signals output from each ultrasound element when an array of multiple ultrasound elements receives ultrasound reflected from an imaging target that has received transmitted ultrasound. The ultrasound image generating unit receives received signals from each of the multiple ultrasound elements from the memory, and generates an image for a predetermined imaging range by performing receive beamforming processing using a delay time set based on the sound speed for beamforming. The sound speed variation image generating unit causes the ultrasound image generating unit to generate multiple types of images in which the sound speed for beamforming is changed to multiple types, or generates the images based on the data of the images generated by the ultrasound image generating unit. The coherence index calculation unit finds the change in signal strength in the direction of the sound speed for beamforming by arranging the signal strengths of pixels at corresponding positions between the multiple types of images generated by the sound speed change image generation unit in the order of the sound speed for beamforming, and calculates a coherence index that represents the coherence of the multiple received signals used in the beamforming of the pixel based on the change in signal strength found.

According to the present invention, it is possible to calculate the coherence index of the received signal for each pixel with a small amount of calculation, and obtain high-quality ultrasound images.

FIG. 1 is a block diagram showing the configuration of an ultrasound imaging apparatus according to a first embodiment. FIG. 1A is an explanatory diagram showing waveforms of signals obtained by delaying received signals by a delay time calculated using an optimal sound speed and then adding them together, and a B image generated using the waveforms; FIG. 1B is an explanatory diagram showing waveforms of signals obtained by delaying received signals by a delay time calculated using a sound speed slower than the optimal sound speed and then adding them together, and a B image generated using the waveforms. (a) and (b) are explanatory diagrams showing a three-dimensional data set generated by the sound speed change image generation unit 54 of embodiment 1 and the processing of the coherence index calculation unit 55, and (c) is a graph showing the change in brightness of corresponding pixels extracted by the coherence index calculation unit 55 due to the sound speed c. 1A is a graph showing the relationship between the luminance and sound speed of a pixel with high coherence according to the first embodiment, and FIG. 1B is a graph showing the relationship between the luminance and sound speed of a pixel with low coherence according to the first embodiment. 1A and 1B are explanatory diagrams showing the processing of the average sound speed estimation unit 56 of the first embodiment, in which (a) is a diagram showing the distribution of the coherence index within the ROI 71, (b) is a diagram showing the distribution of the signal intensity within the ROI, and (c) is a graph showing the relationship between the sum or average value (focus index) of the product of the coherence index and the signal intensity within the ROI calculated by the average sound speed estimation unit 56 and the sound speed. 4 is a flowchart showing the operation of the ultrasound imaging apparatus according to the first embodiment. FIG. 11 is a block diagram showing the configuration of an ultrasound imaging apparatus according to a second embodiment. 11A and 11B are explanatory diagrams showing the processing of the average sound speed estimation unit 56 of the second embodiment, in which (a) is a diagram showing the distribution of coherence indices within an ROI 71, and (b) is a diagram showing the distribution of signal intensity within the ROI of the sound speed at which the maximum signal intensity is obtained. 10 is a flowchart showing the operation of an ultrasonic imaging apparatus according to a second embodiment. FIG. 11 is a block diagram showing the configuration of an ultrasound imaging apparatus according to a third embodiment. 13A and 13B are explanatory diagrams showing the processing of the image processing unit 59 of the third embodiment, in which (a) is a diagram showing the distribution of coherence indices in an image, (b) is a diagram showing the distribution of signal intensities in an image, and (c) is a diagram showing the distribution of signal intensities after weighting with the coherence indices. 11 is a flowchart showing the operation of an ultrasonic imaging apparatus according to a third embodiment. FIG. 13 is a block diagram showing the configuration of an ultrasound imaging apparatus according to a fourth embodiment. 10 is a flowchart showing the operation of an ultrasonic imaging apparatus according to a fourth embodiment. FIG. 13 is a block diagram showing the configuration of a sound speed change image generating unit 54 of an ultrasonic imaging apparatus according to a fifth embodiment. 13 is an explanatory diagram showing the process of a sound speed change image generating unit 54 according to the fifth embodiment. 13 is a flowchart showing a processing operation of a sound speed change image generating unit 54 according to the fourth embodiment.

One embodiment of the present invention is described below.

In the present invention, ultrasound is transmitted to the imaging target, the reflected waves are received by an ultrasonic element array, and multiple types of images are generated by performing receive beamforming at multiple different sound speeds. From the characteristic amount of the change in signal strength for each pixel relative to the sound speed, a coherence index for the received signal of each channel (ultrasonic element) used to generate the received signal after receive beamforming that constitutes the signal strength for each pixel is calculated. This makes it possible to extract pixels generated from highly coherent received signals without calculating the correlation within the channel domain data (received signals for each channel).

This makes it possible to estimate the optimal beamforming sound speed (average sound speed) for each pixel of the ultrasound image, which corresponds to the pixel position. In addition, the robustness and accuracy of the average sound speed estimation can be improved. By performing receive beamforming processing using the estimated average sound speed, it becomes possible to capture high-quality images with reduced artifact noise. The optimal beamforming sound speed is called the average sound speed here, in order to distinguish it from the sound speed as the physical property value of tissue. If the physical property sound speed of the imaging target is uniform, the average sound speed and the physical property sound speed will be the same, but since the imaging target usually has multiple physical property sound speeds, the average sound speed in the propagation path of the ultrasound is estimated as the optimal beamforming sound speed. In this embodiment, the average sound speed can be estimated for each pixel of the ultrasound image.

In addition, high-quality images can be generated by synthesizing multiple types of images that have been subjected to receive beamforming at different sound speeds using the coherence of each pixel.

<<Embodiment 1>>
Hereinafter, a description will be given of an ultrasonic imaging apparatus according to embodiment 1. Fig. 1 is a block diagram showing the overall configuration of an ultrasonic imaging apparatus according to this embodiment.

In the first embodiment, after calculating the coherence index for each pixel, the average sound speed of the imaging target is estimated, and subsequent receive beamforming is performed based on the estimated average sound speed.

As shown in FIG. 1, the ultrasound imaging device 30 of this embodiment is configured to include a transmit beamformer 31, a transmit/receive switching unit (T/R) 33, a signal processing device 50, an input unit 35, a control unit 36, and a display unit 37. A probe 20 is connected to the ultrasound imaging device 30. The probe 20 includes a row (array) of ultrasonic elements 21.

The signal processing device 50 includes a channel memory 32 and an ultrasound image generating unit 52 as components for generating an ultrasound image. The signal processing device 50 also includes a sound speed change image generating unit 54, a coherence index calculating unit 55, and a three-dimensional data set memory 58 as components for calculating a coherence index. The signal processing device 50 also includes an average sound speed estimating unit 56 as a component for estimating the average sound speed from the coherence index.

The transmit beamformer 31 generates a transmit signal and passes it to one or more ultrasonic elements 21 via a transmit/receive switching unit (T/R) 33. The ultrasonic element 21 that receives the transmit signal converts the transmit signal into ultrasonic waves and transmits them to the imaging target 10. The ultrasonic waves reflected by the imaging target 10 reach a row of multiple ultrasonic elements 21, and each of the multiple ultrasonic elements 21 converts the ultrasonic waves into a receive signal (channel RF signal) and outputs it in a time series.

The time-series received signals output by each of the multiple ultrasonic elements 21 are stored in the channel memory 51.

The ultrasound image generating unit 52 includes a receive beamformer 53. The receive beamformer 53 receives receive signals from the channel memory 51 for each of the multiple ultrasonic elements 21, delays the signals by a preset delay time, and then adds them up to focus the signals on a predetermined scan line (receive scanning line). This receive beamforming is performed sequentially for multiple receive focuses set at a predetermined interval on the scan line to obtain a receive signal after receive beamforming.

The ultrasound image generating unit 52 causes the receive beamformer 53 to execute receive beamforming processing for each predetermined scan line. The ultrasound image generating unit 52 generates an ultrasound image by arranging the receive signals after receive beamforming for each scan line generated by the receive beamformer 53.

The delay time is the distance between the receiving focus and the ultrasonic element 21 divided by the set sound speed for receiving beamforming, and is generated by the ultrasonic image generating unit 52 for each receiving focus of the scan line and set in the receiving beamformer 53.

At this time, when the sound speed for beamforming matches the sound speed of the tissue to be imaged, the received signals of each ultrasonic element 21 delayed by the receive beamformer 53 based on the delay time set based on that sound speed are in phase (high coherence) as shown in FIG. 2(a). Therefore, the amplitude of the received signal after addition by the receive beamformer 53 is large. The ultrasound image (B image) generated using the received signal after this receive beamforming has high brightness and small image spread.

On the other hand, when the sound speed for beamforming is slower than the sound speed of the tissue to be imaged, the received signals of each ultrasonic element 21 delayed by the receive beamformer 53 due to the delay time set based on the sound speed are not in phase (lower coherence) as shown in FIG. 2(b) compared to the case of FIG. 2(a). Therefore, the amplitude of the received signal after addition by the receive beamformer 53 is smaller than that in FIG. 2(a) and the signal spread is larger. The ultrasound image (B image) generated using the received signal after receive beamforming in FIG. 2(b) has lower brightness and larger image spread compared to FIG. 2(a).

In this embodiment, we focus on the fact that pixel brightness changes depending on the sound speed for beamforming, as shown in Figures 2(a) and (b), and evaluate the coherence after delay of the received signal used to generate that pixel.

First, the sound speed variation image generating unit 54 causes the ultrasound image generating unit 52 to generate a plurality of types of images in which the sound speed for beamforming is varied into a plurality of types. Specifically, the sound speed variation image generating unit 54 sequentially sets a predetermined range and interval of sound speed for beamforming (for example, a sound speed selected from a range of sound speeds of 1400 to 1650 m/s at intervals of 10 m/s) in the ultrasound image generating unit 52, and sequentially generates an image for each sound speed for beamforming. The ultrasound image generating unit 52 calculates a delay time for each set sound speed for beamforming, and sets it in the receive beamformer 53. As a result, the ultrasound image generating unit 52 sequentially generates a plurality of types of images 61, 62, 63, ... with different sound speeds for beamforming.

Here, the images 61, 62, 63, etc. are two-dimensional planar images in the row direction (x direction) of the ultrasonic elements 21 and the depth direction (z direction) of the imaging target 10, as shown in FIG. 1. The sound speed change image generating unit 54 arranges the multiple types of images 61, 62, 63, etc. in the sound speed direction (c direction) for beamforming to generate a three-dimensional data set, and stores the generated images in the three-dimensional data set memory 58.

As shown in Figures 3(a) to (c), the coherence index calculation unit 55 arranges the signal intensities of pixels 61a, 62a, and 63a at corresponding positions (e.g., coordinates (x,z) = (3,7)) of multiple types of images 61, 62, 62... in the order of sound speed c for beamforming. Specifically, the signal strength may be the amplitude value after detection, or the amplitude value after log compression. In this way, the change in signal strength in the sound speed direction for beamforming (direction c) is obtained, and a coherence index that indicates the coherence of multiple received signals used in the receive beamforming of pixels 61a, 62a, and 63a is calculated based on the change in signal strength obtained.

Specifically, as shown in FIG. 3(c), the coherence index calculation unit 55 generates a graph showing the change in signal intensity of corresponding pixels 61a, 62a, 63a... in multiple types of images 61, 62, 63... with different sound velocities for beamforming, with the horizontal axis being the sound velocity c for beamforming and the vertical axis being the magnitude of signal intensity. The coherence index calculation unit 55 obtains a curve 70 in this graph that shows the change in signal intensity.

As shown in Fig. 4(a), the inventors have found that the curve 70 has a feature that, when the pixels 61a, 62a, 63a, etc. are pixels that image real signals with high coherence such as structures in the imaging target 10, the amplitude is maximum at the optimal sound speed (the sound speed of the tissue at the pixel position of the imaging target 10), the curve has an upward convex shape (mountain shape), and can be relatively well approximated by a second-order polynomial. That is, when the curve 70 is approximated by a second-order polynomial, the second-order coefficient is a negative value, and the coefficient of determination ^R2 in the multiple regression analysis is high. The coefficient of determination ^R2 is calculated by formula (1).

On the other hand, as shown in Fig. 4B, when the pixels 61a, 62a, 63a, etc. are pixels that image signals with low coherence such as artifact components such as side lobes and noise components, the amplitude is low at the optimal sound velocity (sound velocity of the tissue at the pixel position of the imaging target 10), and a complex change pattern is observed, such as a downward convex shape (valley type) and a constriction. Therefore, the curve 70 cannot be well approximated by a second-order polynomial. In other words, when the curve 70 is approximated by a second-order polynomial, the second-order coefficient is a positive value and the coefficient of determination ^R2 in the multiple regression analysis is low.

The coherence index calculation unit 55 then calculates at least one of the following features: whether the curve 70 has an upward convex shape, the amount of change in the curve, the accuracy of approximation of the curve to a predetermined polynomial, and the extreme values of the curve, and calculates a coherence index from one or more of the features.

平均音速推定部５６は、コヒーレンス指標算出部５５が算出したコヒーレンス指標と、ビームフォーミング用音速ｃが異なる複数種類の画像６１，６２、６３等の画素６１ａ，６２ａ、６３ａ・・・の信号強度とを用いて、画素６１ａ，６２ａ、６３ａの位置に対応する撮像対象１０の平均音速、即ち最適なビームフォーミング音速を推定する。 As an example, the coherence index calculation unit 55 calculates the coherence index by the following (Equation 2) and (Equation 3). In (Equation 2), a variable h is prepared, and when the second-order coefficient in the second-order polynomial approximation is 0 or more, the variable h is set to 0. When the second-order coefficient in the second-order polynomial approximation is negative, that is, when the shape is upwardly convex (mountain-shaped), the coefficient of determination ^R2 calculated by (Equation 1) in the second-order polynomial approximation is set to the value of the variable h. Then, using the variable h, the coherence index _WCF is calculated using a weighting function such that the output increases from 0 to 1 as the variable h increases from 0 to 1, as shown in (Equation 3).

The average sound speed estimation unit 56 uses the coherence index calculated by the coherence index calculation unit 55 and the signal intensities of pixels 61a, 62a, 63a, etc. of multiple types of images 61, 62, 63, etc. having different beamforming sound velocities c to estimate the average sound speed of the imaged object 10 corresponding to the positions of the pixels 61a, 62a, 63a, i.e., the optimal beamforming sound speed.

Explains how to estimate the average sound speed.

As shown in FIG. 1, the coherence index calculation unit 55 calculates the coherence index for each of a plurality of pixels included in a predetermined ROI 71 within the imaging range (range of images 61, 62, and 63) of the imaging target 10. As a result, a distribution of the coherence index within the ROI 71 can be obtained, for example, as shown in FIG. 5(a).

The average sound speed estimation unit 56 calculates the product of the signal intensity and the coherence index of each pixel 61a etc. in the ROI 71 of one image 61, and calculates the sum or average of the calculated products for the ROI 71. This sum or average is called the focus index. The average sound speed estimation unit 56 also calculates the focus index for each corresponding ROI 71 of other images 62, 63 etc. that have different sound speeds c for beamforming.

The average sound speed estimation unit 56 plots the obtained focus index and the sound speed c for beamforming of the image obtained from it as shown in Figure 5 (c), and estimates the sound speed c for beamforming at which the focus index is maximized as the average sound speed of the region of the imaging target 10 corresponding to the ROI 71.

The average sound speed estimation unit 56 sets the estimated average sound speed as the sound speed c for beamforming for the ultrasound image generation unit 52. As a result, the average sound speed estimation unit 56 causes the ultrasound image generation unit 52 to perform receive beamforming for subsequent imaging using the sound speed c for beamforming that matches the average sound speed of the ROI 71.

The operation of each part of the ultrasound imaging device including the signal processing device 50 of this embodiment will be described below with reference to the flowchart in Figure 6.

The signal processing device 50 is configured with a computer or the like equipped with a processor such as a CPU (Central Processing Unit) or a GPU (Graphics Processing Unit) and a memory, and the functions of each of the sections 52 to 56 of the signal processing device 50 are realized by software when the CPU reads and executes a program stored in the memory. The signal processing device 50 can also be realized in part or in whole by hardware. For example, the signal processing device 50 can be configured using a custom IC such as an ASIC (Application Specific Integrated Circuit) or a programmable IC such as an FPGA (Field-Programmable Gate Array), and the circuit can be designed to realize the functions of each section of the signal processing device 50.

First, the user issues an instruction to perform imaging via the input unit 35, calculates a coherence index, estimates the average sound speed, and presses button 35a if the estimated average sound speed is to be set in the receive beamformer 53. When button 35a is pressed, the control unit 36 causes each unit to execute the flow in FIG. 6.

(Step 201)
First, the transmission beamformer 31 transmits ultrasonic waves from the ultrasonic elements 21 toward the imaging target 10. Ultrasonic waves reflected or otherwise transmitted from the imaging target 10 are converted into reception signals (channel RF signals) by the row of ultrasonic elements 21. The channel memory 51 stores the reception signals from the multiple ultrasonic elements 21, respectively.

(Step 202)
The receive beamformer 53 reads out the receive signal (channel RF signal) for each ultrasonic element 21 from the channel memory 51, and performs phasing and summing by a known receive beamforming method such as a delay summation method or a Fourier phasing method, using a delay time calculated from a preset sound speed c. As a result, a post-beamforming signal is obtained for a plurality of receive focal points along one or more scan lines set in the imaging range.

(Step 203)
The receive beamformer 53 repeats step 202 above until all beamformed signals required to generate one frame of an image are obtained.

(Step 204)
The ultrasound image generator 52 generates an image by arranging the beamformed signals of the scan lines for one frame. The sound speed variation image generator 54 receives the generated image and stores it in the three-dimensional data set memory 58.

(Step 205)
The sound speed variation image generator 54 changes the sound speed c for receive beam forming and sets it in the ultrasound image generator 52. The ultrasound image generator 52 calculates a delay time using the changed sound speed c for receive beam forming and sets it in the receive beam former 53.

(Step 206)
The receive beamformer 53 repeats steps 202 to 204 using the changed sound speed c for receive beamforming to generate an image using the changed sound speed c for receive beamforming. The sound speed variation image generator 54 receives the generated image and stores it in the three-dimensional data set memory 58. This is repeated until all types of sound speeds c for receive beamforming have been set. As a result, the ultrasound image generator 52 generates multiple types of images 61, 62, 63, ... with different sound speeds c for receive beamforming.

The sound speed change image generation unit 54 receives the generated images 61, 62, 63, etc. from the ultrasound image generation unit 52, arranges them in the sound speed direction (c direction) for receive beamforming, generates a three-dimensional data set, and stores the generated images in the three-dimensional data set memory 58.

(Step 207)
The coherence index calculation unit 55 generates a curve 70 representing the change in signal intensity of pixels 61a, 62a, and 63a aligned in the sound speed direction of the three-dimensional data set within the ROI 71, as shown in Figure 3 (c), and calculates the coherence index using equation (1).
(Step 208)
The average sound speed estimation unit 56 calculates, for each of the images 61, 62, 63, etc., the sum or average value of the products of the signal intensities of the pixels 61a in the ROI 71 and the coherence index as the focus index.
(Step 209)
The average sound speed estimating unit 56 estimates that the beamforming sound speed c at which the focus index is maximized is the average sound speed in the region of the imaging target 10 corresponding to the ROI 71. The average sound speed estimating unit 56 sets the estimated average sound speed as the beamforming sound speed c for the ultrasound image generating unit 52.

By performing steps 201 to 209, the sound speed c for beamforming that matches the average sound speed of the ROI 71 is set in the ultrasound image generating unit 52. The ultrasound imaging device can then perform imaging at the optimal sound speed. This makes it possible to capture high-quality images with reduced artifact noise.

In this embodiment, as shown in FIG. 6, a configuration has been described in which the sound speed c for receive beamforming is changed to generate one image at a time, but the present invention is not limited to this configuration. When the ultrasound image generating unit 52 has a configuration having multiple receive beamformers 53, the sound speed variation image generating unit 54 may set delay times for multiple types of sound speeds c for beamforming in the multiple receive beamformers 53, respectively, and cause them to perform beamforming processing in parallel for the same scan line. This makes it possible to generate images with multiple different types of receive beamform sound speeds c in parallel.

<<Embodiment 2>>
An ultrasonic imaging apparatus according to a second embodiment will be described. Fig. 7 is a block diagram showing the overall configuration of the ultrasonic imaging apparatus according to this embodiment. Fig. 8 is an explanatory diagram showing the processing of the average sound speed estimation unit 56 according to this embodiment. Fig. 9 is a flowchart showing the operation of the ultrasonic imaging apparatus according to this embodiment.

The ultrasound imaging device of the second embodiment has the same configuration and operation as the ultrasound imaging device of the first embodiment, but differs from the first embodiment in that it further includes a maximum signal intensity acquisition sound speed calculation unit 57 as shown in FIG. 7, and in the process in which the average sound speed estimation unit 56 estimates the average sound speed, which is the optimal beamforming sound speed of the imaging target. The following mainly describes the differences.

As shown in FIG. 9, the processing of the ultrasound imaging apparatus of the second embodiment is similar to the flow of FIG. 6 of the first embodiment from steps 201 to 207. As a result, in step 207, the coherence index calculation unit 55 calculates the coherence index for each pixel in the ROI 71. As a result, the distribution of the coherence index within the ROI 71 is obtained, as shown in FIG. 8(a).

(Step 210)
In step 210, the maximum signal intensity acquisition sound velocity calculation unit 57 compares the signal intensity of a pixel (for example, pixel 61a of image 61) in the corresponding ROI 71 of multiple types of images 61, 62, 63, etc., which have different sound velocities for beamforming, with pixels 62a, 63a, etc. at corresponding positions of other images 62, 63, etc. As a result, an image with the maximum signal intensity is selected from the images 61, 62, 63, etc. The sound velocity c for beamforming used when generating the selected image is set as the maximum signal intensity acquisition sound velocity at the position of pixel 61a.

By performing this for each pixel within ROI 71, the maximum signal strength acquisition sound speed calculation unit 57 obtains the distribution of maximum signal strength acquisition sound speeds within ROI 71, as shown in FIG. 5(b).

(Step 211)
The average sound speed estimation unit 56 estimates the average sound speed for each pixel using a weighted maximum signal strength acquisition sound speed obtained by weighting the maximum signal strength acquisition sound speed by the coherence index. Specifically, for example, as shown in Fig. 8, the average sound speed estimation unit 56 obtains the weighted maximum signal strength acquisition sound speed for each pixel in the ROI 71, and the average value of the weighted maximum signal strength acquisition sound speeds is set as the average sound speed.

As a result, the average sound speed estimation unit 56 sets the estimated average sound speed as the sound speed c for beamforming for the ultrasound image generation unit 52.

By performing steps 201 to 207, 210, and 211, the sound speed c for beamforming that matches the average sound speed of the ROI 71 is set in the ultrasound image generating unit 52, so that the ultrasound imaging device can perform subsequent imaging at the optimal sound speed. This makes it possible to capture high-quality images with reduced artifact noise.

In the second embodiment, in steps 207 and 210, the coherence index and the maximum signal strength acquisition sound speed are calculated for all pixels in the ROI 71, but this does not necessarily have to be done for all pixels. The coherence index and the maximum signal strength acquisition sound speed may be calculated for only some pixels, and the weighted maximum signal strength acquisition sound speed may be calculated, and the average value may be used as the average sound speed.

In addition, after determining the coherence index and the maximum signal strength acquisition sound speed for only some of the pixels in the ROI, the distribution of the coherence index and the distribution of the maximum signal strength acquisition sound speed in the ROI may be calculated by an interpolation calculation or the like. Then, the distribution of the maximum signal strength acquisition sound speed in the ROI may be weighted by the distribution of the coherence index, the distribution of the weighted maximum signal strength acquisition sound speed may be determined, and the average value of the distribution of the weighted maximum signal strength acquisition sound speed may be set as the average sound speed.

<<Embodiment 3>>
An ultrasonic imaging apparatus according to a third embodiment will be described. Fig. 10 is a block diagram showing the overall configuration of the ultrasonic imaging apparatus according to this embodiment. Fig. 11 is an explanatory diagram showing the processing of an image processing unit 59 according to this embodiment. Fig. 12 is a flowchart showing the operation of the ultrasonic imaging apparatus according to this embodiment.

In the third embodiment, images are weighted using the coherence of each pixel to generate high-quality images with reduced noise and artifacts.

The ultrasound imaging device of embodiment 3 has the same configuration and operation as the ultrasound imaging device of embodiment 1, but differs from embodiment 1 in that it has an image processing unit 59 as shown in FIG. 10 instead of the average sound speed estimation unit 56. The following mainly describes the differences.

As shown in FIG. 12, the processing of the ultrasound imaging apparatus of the third embodiment is similar to the flow of FIG. 6 of the first embodiment from steps 201 to 207. As a result, in step 207, the coherence index calculation unit 55 calculates the coherence index for each pixel in the image. However, while in the second embodiment, the coherence index calculation unit 55 calculates the coherence index for each pixel in the ROI 71 to obtain the distribution of the coherence index within the ROI, in the third embodiment, the coherence index is calculated for each pixel in the entire image. As a result, the distribution of the coherence index within the image is obtained as shown in FIG. 11(a).

(Steps 212, 213)
In step 212, the image processing unit 59 generates a processed image using the signal intensity ( FIG. 11B ) of one or more images among the multiple types of images 61, 62, 63, ... with different sound velocities for beamforming, and the distribution of the coherence index of the entire image obtained in step 207. Specifically, the image processing unit 59 weights one of the multiple types of images 61, 62, 63, ... so that the signal intensity of a first region 71a having a high coherence index in the image is greater than the signal intensity of a second region 71b having a lower coherence than the first region 71a, thereby generating an image ( FIG. 11C ). The generated image is displayed on the display unit 37.

According to this embodiment, as shown in FIG. 11(c), it is possible to suppress low-coherence signals and conversely enhance high-coherence signals, thereby suppressing signal strength caused by artifacts such as side lobes and grating lobes. Therefore, it is possible to generate a high-quality image with improved visibility of the real signals of structures within the imaging target 10.

The image to be processed by the image processing unit 59 may be an image having a general prescribed sound speed, such as 1540 m/s, from among the multiple types of images 61, 62, 63, etc. Alternatively, an image having an average sound speed estimated within a predetermined ROI 71 using the method of embodiment 1 or embodiment 2 may be used.

<<Embodiment 4>>
An ultrasonic imaging apparatus according to a fourth embodiment will be described below. Fig. 13 is a block diagram showing the overall configuration of the ultrasonic imaging apparatus according to this embodiment. Fig. 14 is a flowchart showing the operation of the ultrasonic imaging apparatus according to this embodiment.

In the fourth embodiment, multiple images are weighted and then added using pixel-by-pixel coherence to generate a high-quality image with reduced noise and artifacts.

The ultrasound imaging device of the fourth embodiment has the same configuration and operation as the ultrasound imaging device of the third embodiment, but differs from the third embodiment in that it further includes the maximum signal intensity acquisition sound speed calculation unit 57 described in the second embodiment. The following mainly describes the differences.

As shown in FIG. 14, the processing of the ultrasound imaging apparatus of the fourth embodiment is similar to the flow of FIG. 6 of the first embodiment from steps 201 to 207. As a result, in step 207, the coherence index calculation unit 55 calculates the coherence index for each pixel in the image. As a result, the distribution of the coherence index is obtained.

(Step 210)
In step 210, the maximum signal strength acquisition sound speed calculation unit 57 compares the signal strength of a corresponding pixel (for example, pixel 61a of image 61) of multiple types of images 61, 62, 63... having different sound speeds for beamforming with pixels 62a, 63a... at corresponding positions of other images 62, 63.... As a result, an image with the maximum signal strength is selected from the images 61, 62, 63.... The sound speed c for beamforming used when generating the selected image is set as the maximum signal strength acquisition sound speed at the position of pixel 61a.

By performing this for each pixel in the image, the maximum signal strength acquisition sound speed calculation unit 57 obtains the distribution of maximum signal strength acquisition sound speeds.

(Step 212)
The image processing unit 59 performs weighted addition of the signal intensities of pixels 61a, 62a, 63a... at corresponding positions in a plurality of types of images 61, 62, 63... with different sound velocities for beamforming. At this time, the image processing unit 59 sets the weight of the signal intensity of the image in which the maximum signal strength acquisition sound speed is the sound speed for beamforming to be larger for pixels with a larger coherence index. This allows the weight of the pixel with the maximum signal strength acquisition sound speed to be set large, and the weight of the pixel with a large coherence index among them to be set large, and the signal intensities of the corresponding pixels 61a, 62a, 63a... of the plurality of images 61, 62, 63... to be added.

This allows the signal strengths of multiple images to be added together, improving robustness compared to processing a single image. It also makes it possible to suppress low-coherence signals and, conversely, enhance high-coherence signals, suppressing signal strength caused by artifacts such as side lobes and grating lobes, and generating high-quality images with improved visibility of the actual signals of structures within the imaging target 10.

In the fourth embodiment, in addition to the maximum signal strength/obtainable sound speed calculation unit 57, a minimum signal strength/obtainable sound speed calculation unit may be further provided.
The minimum signal strength acquisition sound speed calculation unit compares the signal strengths of pixels 61a, 62a, 63a... at corresponding positions in multiple types of images 61, 62, 63... with different sound speeds for beamforming between the multiple types of images 61, 62, 63..., and selects the image with the minimum signal strength. The minimum signal strength acquisition sound speed calculation unit obtains the sound speed for beamforming of the selected image as the minimum signal strength acquisition sound speed at the pixel position.

In this case, when weighting, the image processing unit 59 sets weights such that for pixels with a coherence value greater than a predetermined value, the weight of the signal strength of an image in which the maximum signal strength acquisition sound speed is the sound speed for beamforming is greater than the weight of the signal strength of an image in which the minimum signal strength acquisition sound speed is the sound speed for beamforming. Also, the image processing unit 59 sets weights such that for pixels with a coherence value smaller than a predetermined value, the weight of the signal strength of an image in which the minimum signal strength acquisition sound speed is the sound speed for beamforming is greater than the weight of the signal strength of an image in which the maximum signal strength acquisition sound speed is the sound speed for beamforming.

In this way, by further using the minimum signal strength acquisition sound speed for weighting, it is possible to set weighting more accurately and suppress noise, etc., than when only the minimum signal strength acquisition sound speed is used.

<<Embodiment 5>>
An ultrasonic imaging device according to a fifth embodiment will be described. Fig. 15 is a block diagram of a sound speed variation image generating unit 54 of the ultrasonic imaging device according to the present embodiment. Fig. 16 is a diagram for explaining the processing of the sound speed variation image generating unit 54. Fig. 17 is a flow chart showing the processing of generating an image by calculation in the sound speed variation image generating unit 54.

In the first to fourth embodiments, the sound speed variation image generating unit 54 is configured to cause the ultrasound image generating unit 52 to generate a plurality of types of images 61, 62, 63, etc., in which the sound speed for beamforming is varied to a plurality of types. In the fifth embodiment, the sound speed variation image generating unit 54 generates a plurality of types of images 61, 62, 63, etc., in which the sound speed for beamforming is varied to a plurality of types, by calculation, based on the data of an image of one type of sound speed for beamforming generated by the ultrasound image generating unit 52.

Therefore, in the fifth embodiment, as shown in FIG. 15, the sound speed change image generating unit 54 includes a conversion unit 41, a remapping processing unit 42, and a reconversion unit 43.

The calculation process of the sound speed change image generating unit 54 will be explained using the flow in Figure 17. The calculation process of the sound speed change image generating unit 54 is a known technique, so it will be explained briefly here.

(Step 301)
The sound speed variation image generating unit 54 receives data of the first image generated using the first delay time determined based on the first sound speed C ₀ for beamforming from the ultrasonic image generating unit 52 (FIG. 16(a)).

(Step 302)
The converter 41 converts the data of the first image into first wavenumber space data in wavenumber space ( FIG. 16( b )).

The first image is a two-dimensional image in the x-z plane, with the row direction of the ultrasonic elements 21 being the x direction and the depth direction of the imaging target 10 being the z direction. The conversion unit 41 converts this into data in a two-dimensional wave number space with the wave number (kx) direction in the x direction and the wave number (kz) direction in the z direction as two axes.

(Step 303)
The remapping processing unit 42 processes the first wave number space data to generate data equivalent to second wave number space data obtained by converting a second image obtained when the received signal is processed with a second delay time determined based on the sound speed for second beamforming.

The second wave number space data is wave number space data obtained when the received signal (channel RF data) is processed with a second delay time _T2 (e.g., _T2 = L/ _C1 ) determined based on the distance L between the receiving focal point and the ultrasonic element ₂₁ and the second sound speed C1.

The coordinates ( _kx1 , _kz1 ) of the second wavenumber space data have a predetermined relationship expressed using the coordinates ( _kx0 , _kz0 ) of the data constituting the first wavenumber space data and the sound velocities _C0 , _C1 . The sound speed change image generating unit 54 stores in advance in the table storage unit 44, for example in the form of a table, the relationship between the coordinates ( _kx0 , _kz0 ) of the first wavenumber space data and the corresponding coordinates ( _kx1 , _kz1 ) when transformed into the coordinates of the data when phased at the second delay time T2 of the sound speed C1 (see FIG _. 15).

The remapping processing unit 42 determines, by interpolation or the like, data values of sampling coordinates (kx ₁ , kz ₁ ) (circular coordinates in FIG. 16( c )) shifted by a distance determined in advance based on the first sound speed C ₀ and the second sound speed C ₁ from the sampling coordinates (kx ₀ , kz ₀ ) of the data constituting the first wavenumber space data ( FIG. 16( b )). When the data determined by the interpolation or the like is represented in wavenumber space at the second sound speed (C ₁ ), it becomes equally spaced sampling ( FIG. 16( d )).

Therefore, the remapping processing unit 42 refers to the table storage unit 44, reads out the coordinates ( _kx1 , _kz1 ) and coordinates ( _kx0 , _kz0 ), calculates the data value of the coordinates ( _kx1 , _kz1 ) of the first wavenumber space data by interpolation (Figure 16(c)), and generates the second wavenumber space data corresponding to the sound speed _C1 (Figure 16(d)).

(Step 304)
The reconversion unit 43 generates a second image corresponding to the second sound speed _C1 by inversely converting data equivalent to the second wave number space data generated by the remapping processing unit 42 ( FIG. 16( e )). The obtained second image is data equivalent to an image obtained by phasing the received signal (channel RF data) by the second delay time T2 determined based on the second sound speed _C1 .

(Step 305)
The sound speed variation image generating unit 54 repeats steps 303 and 304 until images are generated for all predetermined sound speeds _C2 , _C3 , .... Note that the table storage unit 44 also stores information on coordinates to be calculated by interpolation in the first wave number space data for a plurality of sound speeds _C2 , _C3 , ... other than the second sound speed _C1 .

The sound speed change image generation unit 54 stores the generated images corresponding to each sound speed in the three-dimensional data set memory 58.

As described above, according to the ultrasound imaging device of the fifth embodiment, by processing the generated image, an image equivalent to an image in which the sound speed during receive beamforming is changed can be generated with a small amount of calculation. Therefore, by performing subsequent processing in the same manner as in the first to fourth embodiments, artifacts such as side lobes and grating lobes can be suppressed by estimating the average sound speed or by image processing, and a high-quality image can be generated in which the visibility of the real signals of structures within the imaging target 10 is improved.

As described above, according to the first to fifth embodiments of the present invention, highly coherent pixels can be extracted on a pixel-by-pixel basis without performing correlation calculations on the received signals between channels, which improves the robustness and accuracy of sound speed estimation and enables high-quality imaging with reduced artifact noise.

10...imaging target, 20...probe, 21...ultrasonic element, 30...ultrasonic imaging device, 31...transmission beamformer, 32...channel memory, 35...input unit, 35a...button, 36...control unit, 37...display unit, 41...conversion unit, 42...remapping processing unit, 43...reconversion unit, 44...table storage unit, 50...signal processing device, 51...channel memory, 52...ultrasonic image generating unit, 53...receiving beamformer, 54...sound speed change image generating unit, 55...coherence index calculation unit, 56...average sound speed estimation unit, 57...maximum signal intensity acquisition sound speed calculation unit, 58...three-dimensional data set memory, 59...image processing unit, 61, 62, 63...image, 61a, 62a, 63a...pixel, 70...curve, 71...ROI, 71a...first region, 71b...second region

Claims (6)

A memory for storing a received signal output from each of a plurality of ultrasonic elements, the received ultrasonic waves being reflected by an imaging target to which the ultrasonic waves are transmitted;
An ultrasonic image generating unit that receives the reception signals for each of the ultrasonic elements from the memory and performs reception beamforming processing using a delay time set based on a sound velocity for beamforming to generate an image for a predetermined imaging range;
A sound speed change image generating unit that generates a plurality of types of the image in which the sound speed for beam forming is changed into a plurality of types by causing the ultrasonic image generating unit to generate the plurality of types of the image in which the sound speed for beam forming is changed into a plurality of types by calculation based on the data of the image generated by the ultrasonic image generating unit;
A coherence index calculation unit that obtains a change in the signal strength in the sound speed direction for beamforming by arranging the signal strengths of pixels at corresponding positions between the multiple types of images generated by the sound speed change image generation unit in the order of the sound speed for beamforming, calculates a feature value of the obtained change in signal strength, and outputs a coherence index value representing the coherence of the multiple received signals used in beamforming of the pixel based on the feature value;
An average sound speed estimation unit;
The average sound speed estimating unit estimates an average sound speed, which is an optimal beamforming sound speed of the imaging target corresponding to the position of the pixel where the coherence index is obtained, using the value of the coherence index and the signal intensities of the multiple types of images having different sound speeds for beamforming, sets the estimated average sound speed as the sound speed for beamforming for the ultrasound image generating unit, and executes subsequent imaging;
the coherence index calculation unit calculates a value of the coherence index for each of a plurality of pixels included in a predetermined ROI within the imaging range;
The average sound speed estimation unit is
A process of calculating a product of the signal intensity of the pixel for which the coherence index is calculated and the value of the coherence index, and calculating a sum or average value of the product over the ROI as a focus index, is performed for each of the ROIs corresponding to the plurality of types of images having different sound velocities for beamforming;
An ultrasonic imaging apparatus comprising: determining the image in which the obtained focus index is maximized; and estimating the beamforming sound speed of the determined image as the average sound speed. 2. The ultrasonic imaging apparatus according to claim 1, wherein the feature amount is a value representing an approximation accuracy when the change in the signal intensity is approximated to a polynomial with a negative quadratic coefficient,
The coherence index calculation unit outputs a larger value as the coherence index value, assuming that the higher the approximation accuracy, the greater the coherence of the multiple received signals used in beamforming of the pixel. A memory for storing a received signal output from each of a plurality of ultrasonic elements, the received ultrasonic waves being reflected by an imaging target to which the ultrasonic waves are transmitted;
An ultrasonic image generating unit that receives the reception signals for each of the ultrasonic elements from the memory and performs reception beamforming processing using a delay time set based on a sound velocity for beamforming to generate an image for a predetermined imaging range;
A sound speed change image generating unit that generates a plurality of types of the image in which the sound speed for beam forming is changed into a plurality of types by causing the ultrasonic image generating unit to generate the plurality of types of the image in which the sound speed for beam forming is changed into a plurality of types by calculation based on the data of the image generated by the ultrasonic image generating unit;
A coherence index calculation unit that obtains a change in the signal strength in the sound speed direction for beamforming by arranging the signal strengths of pixels at corresponding positions between the multiple types of images generated by the sound speed change image generation unit in the order of the sound speed for beamforming, calculates a feature value of the obtained change in signal strength, and outputs a coherence index value representing the coherence of the multiple received signals used in beamforming of the pixel based on the feature value;
An average sound speed estimation unit;
Maximum signal strength acquisition and sound speed calculation unit
The average sound speed estimating unit estimates an average sound speed, which is an optimal beamforming sound speed of the imaging target corresponding to the position of the pixel where the coherence index is obtained, using the value of the coherence index and the signal intensities of the multiple types of images having different sound speeds for beamforming, sets the estimated average sound speed as the sound speed for beamforming for the ultrasound image generating unit, and executes subsequent imaging;
The maximum signal intensity acquisition sound velocity calculation unit selects the image in which the signal intensity is maximum by comparing the signal intensity of the pixel in the corresponding ROI of the plurality of types of the images having different sound velocities for beamforming between pixels at corresponding positions of the plurality of types of the images, and obtains the sound velocity for beamforming of the selected image as the maximum signal intensity acquisition sound velocity at the position of the pixel,
The coherence index calculation unit calculates the coherence index for the pixels in the ROI,
The average sound speed estimating unit estimates the average sound speed using a weighted maximum signal strength acquisition sound speed obtained by weighting the maximum signal strength acquisition sound speed by the calculated coherence index value,
The coherence index calculation unit and the maximum signal intensity acquisition sound speed calculation unit calculate the coherence index value and the maximum signal intensity acquisition sound speed for the plurality of pixels in the ROI,
The average sound speed estimation unit obtains the weighted maximum signal intensity acquisition sound speed for the plurality of pixels in the ROI, and sets the average value of the weighted maximum signal intensity acquisition sound speed as the average sound speed;
the coherence index calculation unit calculates a distribution of the coherence index values within the ROI;
The maximum signal intensity acquisition sound speed calculation unit calculates a distribution of the maximum signal intensity acquisition sound speed within the ROI,
The average sound speed estimation unit obtains a distribution of the weighted maximum signal strength acquired sound speed by weighting the distribution of the maximum signal strength acquired sound speed within the ROI by the distribution of the coherence index values within the ROI, and determines the average value of the distribution of the weighted maximum signal strength acquired sound speed as the average sound speed. An ultrasound imaging device characterized by the above. A memory for storing a received signal output from each of a plurality of ultrasonic elements, the received ultrasonic waves being reflected by an imaging target to which the ultrasonic waves are transmitted;
An ultrasonic image generating unit that receives the reception signals for each of the ultrasonic elements from the memory and performs reception beamforming processing using a delay time set based on a sound velocity for beamforming to generate an image for a predetermined imaging range;
A sound speed change image generating unit that generates a plurality of types of the image in which the sound speed for beam forming is changed into a plurality of types by causing the ultrasonic image generating unit to generate the plurality of types of the image in which the sound speed for beam forming is changed into a plurality of types by calculation based on the data of the image generated by the ultrasonic image generating unit;
A coherence index calculation unit that obtains a change in the signal strength in the sound speed direction for beamforming by arranging the signal strengths of pixels at corresponding positions between the multiple types of images generated by the sound speed change image generation unit in the order of the sound speed for beamforming, calculates a feature value of the obtained change in signal strength, and outputs a coherence index value representing the coherence of the multiple received signals used in beamforming of the pixel based on the feature value;
An image processing unit,
The image processing unit generates a processed image using the signal intensity of one or more images among the plurality of types of images having different sound velocities for beamforming and the value of the coherence index,
the coherence index calculation unit calculates values of the coherence index for a plurality of the pixels included in an imaging range to obtain a distribution of the coherence index within the image;
The image processing unit performs image processing for one or more images among the multiple types of images having different sound velocities for beamforming such that a signal intensity of a first region in which the coherence index value indicates high coherence is greater than a signal intensity of a second region in which the coherence index value indicates lower coherence than the first region, thereby generating the processed image;
The ultrasonic imaging device according to claim 1, wherein the image processing unit generates the processed image by weighting and adding signal intensities of the pixels at corresponding positions of a plurality of types of the images having different sound velocities for beamforming. 5. The ultrasonic imaging apparatus according to claim 4 , further comprising a maximum signal intensity acquisition sound velocity calculation unit,
The maximum signal strength acquisition sound speed calculation unit compares the signal strength of the pixel at the corresponding position of the multiple types of the images having different sound speeds for beamforming between the multiple types of the images, selects the image in which the signal strength is maximum, and obtains the sound speed for beamforming of the selected image as the maximum signal strength acquisition sound speed at the position of the pixel, thereby obtaining a distribution of the maximum signal strength acquisition sound speed,
The image processing unit sets the weighting of the signal intensity of the image, in which the maximum signal intensity acquisition sound speed during the weighting is the sound speed for beamforming, so that the weighting is larger for pixels with greater coherence indicated by the coherence index. 6. The ultrasonic imaging apparatus according to claim 5 , further comprising a minimum signal intensity acquisition sound velocity calculation unit,
The minimum signal strength acquisition sound speed calculation unit compares the signal strength of the pixel at the corresponding position of the plurality of types of the images having different sound speeds for beamforming between the plurality of types of the images, selects the image in which the signal strength is minimum, and obtains the sound speed for beamforming of the selected image as the minimum signal strength acquisition sound speed at the position of the pixel,
The image processing unit is characterized in that, during the weighting, for pixels having a coherence value greater than a predetermined value, the signal strength weight of the image in which the maximum signal strength acquisition sound speed is the sound speed for beamforming is greater than the signal strength weight of the image in which the minimum signal strength acquisition sound speed is the sound speed for beamforming, and for pixels having a coherence value smaller than a predetermined value, the signal strength weight of the image in which the minimum signal strength acquisition sound speed is the sound speed for beamforming is greater than the signal strength weight of the image in which the maximum signal strength acquisition sound speed is the sound speed for beamforming.

Images